*The "Great American Solar Eclipse" is now just about one month away*

OverviewThere is now just a little over a month to go to the first coast-to-coast total solar eclipse in the US since 1918. On August 21st, the moon will pass between the sun and Earth and the result will be a 67-mile wide shadow that will cross the country from Oregon to South Carolina. During a total solar eclipse (and of course depending on the local weather conditions), the sky gets deep twilight blue, temperatures drop noticeably, the winds should die down, bright stars and planets come out, animals and birds may behave strangely like it’s the end of the day, and it can become strangely silent. A total solar eclipse provides a great opportunity for scientists to learn more about the sun's atmosphere as well as our own.

General DiscussionFrom all first-hand accounts, there is a major difference in the impact of viewing a partial solar eclipse versus witnessing a total solar eclipse. A partial solar eclipse (which is what this will be in the DC, Philly and NYC metro regions) has been described as a "run-of-the-mill" astronomical event whereas a total solar eclipse is referred to as a "must see" and a "once-in-a-lifetime" opportunity.

Total solar eclipses occur somewhere on Earth every year or so, but generally cast their shadows over oceans or remote land masses. The last time a part of the contiguous US saw a total solar eclipse was in 1979 and the next total solar eclipse on US soil will take place in April 2024. The last total solar eclipse in the contiguous United States, 38 years ago, only clipped the northwestern United States, mostly rural areas of Washington, Oregon, Idaho, Montana and North Dakota. And on that particular day, Feb. 26, 1979, it was cold and dreary in the Northwest, and most people in the path of totality did not even see the eclipse due to clouds and rain.

On August 21, 2017, the moon will move in between the Earth and the sun causing the moon to cast an umbral shadow over the planet and millions of Americans will witness a total solar eclipse. Everyone in the continental US outside of the "totality zone" will witness a partial eclipse of at least 48% caused by the moon's penumbral shadow. The 67-mile wide path of the moon’s umbral shadow in the "totality zone" will begin in the northern Pacific and cross the U.S. from west-to-east through parts of the following states: Oregon, Idaho, Montana, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, North Carolina, Georgia, and South Carolina. It will pass directly over cities such as Salem, Ore., Idaho Falls, Lincoln, Neb., Kansas City, Nashville, and Columbia and Charleston, S.C. Places within a one- or two-hour drive of the eclipse include Portland, Ore., Boise, Cheyenne, Rapid City, Omaha, Neb., Topeka, St. Louis, Louisville, Knoxville, Chattanooga, Atlanta and Charlotte. At any given location, the total eclipse will last for around 2 or 3 minutes turning day into a dark twilight and the total event will take about three hours from start to finish.

In the DC-to-Philly-to-New York City corridor, the eclipse will be partial and its duration will be about 2 hours and 40 minutes. The beginning of the solar eclipse in this corridor will be around 1:20 PM (ET), the time of maximum coverage of the sun around 2:45 PM (ET), and the ending time around 4:00 PM (ET). The percentage of totality will be roughly 81% in the DC metro region, 75% in and around Philly, and 72% in and around the NYC metro region.

On August 21, 2017, the moon will pass between Earth and the sun in a total solar eclipse that will be visible on a path from Oregon to South Carolina across the continental United States. Map courtesy NASA

Scientific value of a total solar eclipseThe atmosphere of the sun is composed of several layers, mainly the photosphere, the chromosphere, and the corona. It is in these outer layers that the sun’s energy which has bubbled up from the interior is detected as sunlight. The lowest layer of the sun’s atmosphere is the photosphere (source region of solar flares) and it is about 300 miles thick. The next layer is the chromosphere which emits a reddish glow as super-heated hydrogen burns off. This red rim can only be seen during a total solar eclipse as at other times, light from the chromosphere is usually too weak to be seen against the brighter photosphere.

The third layer of the sun’s atmosphere is the corona and this too can only be seen during total solar eclipses - not even a 99 percent eclipse will reveal the sun’s corona. The corona appears as white streamers or plumes of ionized gas that flow outward into space. Temperatures in the corona can get as high as 3.5 million degrees F making the region much hotter than the solar surface which is just 11,000 degrees F or so. How the corona gets so hot has puzzled scientists for decades and solar scientists aim to gather some useful data during this upcoming total solar eclipse.

The first recorded observation of the solar corona occurred during the total eclipse of December 22nd in the year 968 A.D. Byzantine historian Leo Diaconus observed the event from Constantinople and wrote this:"...at the fourth hour of the day ... darkness covered the Earth and all the brightest stars shone forth. And it was possible to see the disk of the sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk."

In addition to the sun’s atmosphere, a total solar eclipse can provide useful information on coronal mass ejections (CME) – material from the sun that is spewed into space via enormous explosions. The first record of a CME in progress was made during the total solar eclipse of July 18th, 1860. Furthermore, scientists can use solar eclipses to study the nature of the Earth’s own atmosphere. Specifically, this event could provide useful data on the Earth’s ionosphere as ultraviolet light and x-rays from the sun can strip electrons from atoms in that part of the atmosphere in a process called ionization. Also, during total solar eclipses scientists can learn about how the presence or absence of sunlight influences radio frequencies.

Temperature drops, “eclipse wind” and "strange silences” in totality regionsTotal solar eclipses have featured noticeable temperature drops as sunlight quickly disappears. Interestingly, it has been found that the temperature drop tends to vary depending on the underlying landscape. Observations have shown that surface temperatures tend to fall less in coastal regions, mountainous areas and in those locations covered in vegetation.

In addition, total solar eclipses often bring about a noticeable drop in surface winds which sometimes is referred to as the “eclipse wind”. Also, this supports the “strange silences” experienced during these events. The wind change appears to be caused by variations to the “boundary layer” in the atmosphere – the area of air which usually separates high-level winds from those on the ground. As temperatures dropped during the total solar eclipse, it appears this boundary layer also fell closer to the surface, meaning the drag on the wind from the land was greater. It is similar to what occurs at sunset, but over a much shorter period of time. In addition to the drop in winds which can contribute to the “strange silences” experienced during total solar eclipses, birds often go silent as skies darken.

This map shows the path of totality overlaid on a statistical map of cloudiness for the month of August where the best places to be are shown in blue. Courtesy spaceweather.com and meteorologists Jay Anderson, Jennifer West

Best viewing places based on climatologyBased on historical data from NASA's Terra and Aqua satellites, the best place to view the total solar eclipse from a statistical viewpoint is in the interior western US (blue areas on map) where fractional cloud cover dips as low as 15%. In general, cloud cover becomes more of a problem east of the Mississippi River and it can be problematic along both immediate coastlines. From a meteorological point of view, one thing to monitor as we approach the August 21st time period will be tropical moisture to see if it could play a role in cloud cover across the southeastern US. The Atlantic Basin tropical season will be well underway by this time and a tropical system could certainly be problematic in terms of cloud cover. Make your plans now and hope for good weather....just a little over a month to go.

In case you miss this one, here is the path of the next total solar eclipse in the US on April 8, 2024

Warning: Never look at the sun without proper eye protection. Looking directly at the sun during a partial eclipse (such as will be the case in DC, Philly, NYC) can cause serious eye damage or blindness.